Group Manager is a implementation of permissions. It allows you to set up permissions for groups, and add users to these groups. The result is you can have multiple tiers of access to commands. This is useful for most Minecraft servers, as it lets you build a community, without it you struggle to balance the right number of admins with users.
Group Manager is offered as a separate module, and is not currently available as a BukkitDev approved project.
See also: Group Manager/Config, Group Manager/Commands, and Group Manager/Developers
Video Tutorial covering a subsection of this page. - There are some minor inaccuracies so where this document disagrees with the video, use this document.